Prognostic markers that can discriminate between aggressive and indolent forms of B cell chronic lymphocytic leukemia (CLL) have provided remarkable insight into the clinical heterogeneity of the disease and are influencing the design of clinical trials, but the technical complexity and variability in assaying these indicators have presented challenges for hospital laboratories. The most significant breakthrough in the field was the discovery that CLL can be segregated into two major subtypes that differ in their clinical aggression according to the degree of somatic hypermutation in the immunoglobulin heavy-chain variable region (IGHV) gene. Individuals with mutated IGHV genes (MT-CLL) have a median survival that is three times longer than patients with unmutated IGHV genes (UM-CLL). Because sequencing IGHV regions is costly and impractical for clinical labs, most hospitals now analyze CD38 and ZAP-70 using flow cytometry given their preferential expression by UM-CLL cells. Although both proteins are significantly associated with a poor prognosis, they are ~25-30% discordant with IGHV mutation status. Furthermore, because ZAP-70 is located in the cytoplasm, its staining can vary and protocols to evaluate it are not yet standardized. The long term goal is to investigate the pathogenesis of CLL. The overall objective of this proposal is to validate the clinical utility of a novel biomarker of CLL prognosis. Our central hypothesis is that Fc receptor-like 2 (FCRL2), a unique indicator of indolent MT-CLL, will more accurately predict IGHV mutation status and clinical progression than other currently employed prognostic markers. This hypothesis is based on: (i) our recently published findings that FCRL2 robustly predicts IGHV mutation status and clinical progression in CLL, (ii) newly developed preliminary data outlined in the Research Strategy, and (iii) independent work by other investigators that supports this hypothesis. The rationale for the proposed research is that optimization and validation of FCRL2 detection on CLL cells will facilitate its wide application and improve prognostic accuracy over existing indicators. We plan to test the central hypothesis by pursuing the following specific aims: 1) Optimize the detection of FCRL2 on CLL cells by flow cytometry and 2) Validate and qualify the prognostic utility of FCRL2 in predicting IGHV mutation status, clinical disease progression, and survival in CLL. In Aim 1, we will optimize newly developed reagents for detecting FCRL2. In Aim 2, we will use these analytical tools to validate the utility of FCRL2 in predicting IGHV mutation status and clinical outcome. The proposed research is innovative, because it focuses on a surface receptor distinctly upregulated on MT-CLL cells that provides the highest concordance with IGHV status. This contribution will be significant because it will advance currently established approaches to prognostication in CLL by improving accuracy, consistency, ease of use, and cost. The use of FCRL2 will change the approach to CLL prognostication at diagnosis, have collective benefits when used with other markers, and is anticipated to ultimately modify approaches to clinical treatment in the most common leukemia in Western countries.